Prostate cancer is the most common malignancy in males and the second leading cause of death from cancer in men. It is estimated that there will be 238,590 new cases of prostate cancer in 2013. The majority of these patients will undergo definitive treatment. However, about 35% are expected to have biochemical recurrence within 10 years. This translates to nearly 70,000 patients a year with PSA recurrence after definitive therapy. Currently, no commercially available molecular imaging agent can effectively localize regional prostate metastases in soft tissue. Talab et al., Radiol Clin North Am; 50(6):1015-1041 (2012). The development of a sensitive and specific method to non-invasively localize prostate cancer in its early stages within the prostate and in local pelvic lymph nodes would profoundly change the workup and management of prostate cancer.
Small molecule inhibitors of prostate specific membrane antigen (PSMA) have shown the potential to be good agents for prostate cancer imaging. PSMA is a type II membrane protein with a very short intracellular domain connected by a single transmembrane helix to a large extracellular domain. Israeli et al., Cancer Res; 53(2):227-230 (1993). PSMA was first identified as the molecular target of the 7E11-C5 antibody which selectively binds LNCaP cells. In addition to its normal expression in the central nervous system, urogenital system, and small bowel, PSMA is over-expressed on prostate cancer cells and tumor neovasculature. A simple, easy to synthesize, and yet potent, urea-based small molecule inhibitor of PSMA was first published in 2001. Kozikowski et al., J Med Chem; 44(3):298-301 (2001). During the last decade, the simple di-amino acid urea compounds first made by Kozikowski et al. have evolved into a myriad of imaging agents for single photon emission tomography (SPECT) and positron emission tomography (PET).
Small molecule PET and SPECT PSMA tracers that have been tested in animals and humans have demonstrated a great advancement compared to antibody-based SPECT imaging with 111In-capromab pendetide. 18F-DCFBC was the first PSMA-targeting PET tracer to be tested in humans. Cho et al., J Nucl Med; 53(12):1883-1891 (2013). In a small five-patient trial, 18F-DCFBC detected lymph node and bone metastases at 2 hr post injection. In a seven-patient phase 1 study of 123I-MIP-1072 and 123I-MIP-1095, the SPECT tracer also demonstrated detection of soft tissue and bone metastases, as well as tumors in the prostate bed. Barrett et al., J Nucl Med; 54(3):380-387 (2013). Afshar-Oromieh et al. tested Glu-NH—CO—NHLys(Ahx)-[68Ga(HBED-CC)] in 37 patients with prostate cancer and demonstrated a (per patient) lesion detection rate of 60% at PSA<2.2 ng/ml and a detection rate of 100% at PSA >2.2 ng/ml. Afshar-Oromieh et al., Eur J Nucl Med Mol Imaging; 40(4):486-495 (2013). All of these early human trials showed good lesion to background contrast at a few hours post injection compared to 111In-capromab penditide images, which need to be acquired 4 days post injection. However, 18F-DCFBC also had elevated liver background and unexpected blood pool retention. 123I-MIP-1072, 123I-MIP-1095, and Glu-NH—CO—NH-Lys(Ahx)-[68Ga(HBEDCC) all showed significant uptake in salivary glands, lacrimal glands, and liver. These organs have no significant expression of PSMA. When the tracer molecules are used as diagnostic agents, the elevated background affects the overall sensitivity of detection. If these agents would be used for therapy, unintended background would increase the overall toxicity of the treatment.
The non-PSMA related background activity exhibited by the current tracers may be due to hydrophobic interactions. Small radiolabeled PSMA tracers contain an aromatic group for convenient radiohalogenation. Many of the imaging agents with bulky NIR fluorophores and radionuclide chelates have long slender hydrophobic linkers that join the fluorophor or metal chelate to the di-amino acid urea moiety. A long linker is necessary because a 20 Å substrate tunnel connects the surface of PSMA with its deep ectodomain. Mesters et al., EMBO J.; 25(6):1375-1384 (2006). Early design efforts with 99mTc tracers demonstrated that there is a minimal linker length needed for proper binding. Banerjee et al., J. Med Chem; 51 (15):4504-4517 (2008). In the available high resolution structures (such as PDB 3D7G and 3D7H), where space allows, only a few crystallographic water molecules are seen within the tunnel. Given the known structure of PSMA, one would expect low binding affinity for RBI-1033, a urea-based PSMA targeting compound containing a bulky 2-5 Å moiety in the substrate tunnel region. Cramer Nucleosides Nucleotides Nucleic Acids; 26(10-12):1471-1477 (2007). The axial dimension of the 2-5 Å moiety greatly exceeds the width of the tunnel.
Surprisingly, RBI1033 exhibits ten times higher affinity toward PSMA than its “parent” urea ligand. The high affinity of RBI-1033 and its derivatives to PSMA suggest that a bulky linker is acceptable in the substrate tunnel. Wang et al., Nucleosides Nucleotides Nucleic Acids; 31(5):432-444 (2012). However, there remains a need for additional compounds useful as PSMA imaging agents.